French President Emmanuel Macron will seek to convince his US counterpart Donald Trump on Thursday to stick to a global deal to combat climate change ahead of a Group of Seven leaders where there is currently no consensus on the matter, diplomats said.

Trump, who doubts climate change is man-made and made a campaign pledge to "cancel" the 2015 Paris Agreement, has postponed a planned decision on whether to stay or leave that had been due before the May 26-27 summit in Italy.

A newcomer to international diplomacy, Macron travels on Thursday to Brussels for a NATO summit before heading to Sicily for the G7 summit that also includes Germany, Britain, Italy, Japan and Canada.

Ahead of NATO, he will meet Trump for the first time for an informal lunch that will immediately test his diplomatic skills given the unpredictable nature of the U.S. leader and France's desire to ensure the U.S. does not renege on the climate deal.

"What's at stake is to be firm on the Paris accord," said a senior French diplomat, adding that Macron would put his case to Trump in the face-to-face talks in the Belgian capital.

"We don't want the U.S. to pull out because it would be a very bad signal and lead others to pull out."

Diplomats said there had been no agreement on these issues ahead of the G7 gathering, meaning the leaders will seek to strike an accord amongst themselves.

"The U.S. did not accept (a common position) during the ministerial talks ... so one of the key issues is to try and obtain the commitment from all the G7 members and the Americans," said a second diplomat.

He added that there was a possibility the Italian presidency could issue a separate statement on climate change if Trump did not endorse the Paris agreement.

"It's strange because normally the final communique is agreed 10 days before, but here we will probably be negotiating overnight between Friday and Saturday."